The present invention relates to compression moulding of high molecular weight polymers, and components made by these moulding processes.
The processing of ultra-high molecular weight polymers into load-bearing components by techniques conventional for thermoplastics is not feasible because of the exceptionally high viscosity of such materials. The polymer manufacturer produces a polymerised powder which then has to be compacted into a continuous solid. One method of compaction is to produce large slabs or rods by melting the powder and compressing or extruding the melt followed by cooling to form the solid. Individual components are then manufactured from this bulk stock by machining.
An alternative is direct net-shape compression moulding of the powder in heated moulds of an appropriate shape. A weighed charge of the powder is poured into the fixed part of the mould and pressure applied through the moving part of the mould. Components of the same dimensions in plan, but of different thickness, can be made by varying the weight of the charge.
In order to ensure the mechanical integrity of components and to maximise their strength and wear resistance, it is essential for the powder to be fully compacted. In addition, for there to be no residual planes of weakness at the original particle boundaries (which give rise to preferential sites for fatigue crack growth), the moulding process must achieve complete homogenisation of the polymer. The process of homogenisation of the polymer may also be described by the terms welding, diffusion, self-diffusion, consolidation, and/or fusion. Complete homogenisation requires two steps to be completed, aided by elevated pressure and temperature. Firstly, there must be complete compaction of the powder, with the particles being deformed and pressed into intimate contact (fully wetted) at the molecular level, with all voids removed. Secondly, the polymer chains must interdiffuse across the particle interfaces, until the entanglements on either side of the interfaces are fully knitted together, and no memory of the original boundaries remains.
For these two processes to be achieved, sufficient time is required for thermal conduction from the heated surfaces of the mould to raise the temperature in the centre of the mould to that required for homogenisation; in addition, this temperature must be maintained for a sufficiently long period of time for homogenisation to take place. Manufacturers do not reveal what pressure, time and temperature cycles are used in the production of their components and these doubtless vary from manufacturer to manufacturer.
Compression moulding techniques find particular application in the manufacture of joint replacement prostheses, such as artificial hip and knee joints.
Over 1 million joint replacements are implanted annually world-wide. Virtually all of them include ultra-high molecular weight polyethylene (UHMWPE) elements which provide a low-friction arthroplasty when articulating with polished metal surfaces.
Direct moulding has certain advantages, when compared to moulding of large slabs or rods, followed by machining. The only external machining marks are those of the mould and it is possible to achieve a highly smooth and glossy surface finish. Moreover, the polymer may be moulded around metallic inserts to produce composite components.
UHMWPE has been the most widely used material for bearing surfaces in total knee and hip joint replacement prostheses since the 1970s, because of its suitable properties of biocompatibility, high impact strength, low friction and high wear resistance (Li, S. and Burstein, A. H., The Journal of Bone and Joint Surgery, Incorporated, 76-A, 1080 (1994)). However, recent research into the microstructure of the material has shown that incomplete consolidation of the UHMWPE powder, resulting in xe2x80x9cfusion defectsxe2x80x9d, is implicated in the failure of the material due to fatigue (Mayor , M. D., Wrona, M., Collier, J. P. and Jensen, R. E., Clinical Orthopaedics and Related Research, 299, 92 (1994)). Cracking and delamination, specifically associated with fatigue failure, have been found in retrieved knee components. A large literature has been produced characterising the morphology of the wear particles, describing the cellular reactions to the particles, defining the effects of oxidisation and irradiation on the mechanical properties of the polymer, describing the results of wear tests in hip and knee simulators in the laboratory and retrieval studies of components removed from patients at revision. To minimise the occurrence of wear, cracking and deformation, there is an urgent need to improve understanding of the factors governing homogenisation and removal of fusion defects, and to develop reliable means of ensuring their absence, by proper engineering of the manufacturing process. In practice, there is a range of prosthesis designs in use, and each design of component is produced in a variety of sizes, to suit the needs of different patients. It is necessary to develop ideal moulding conditions for each size of component.
Therefore, there is a need for a flexible computer-aided engineering methodology, that can be used at the design stage for products to optimise the moulding process for each individual component. There has been virtually no discussion in the scientific community about optimising the moulding/manufacturing process, other than the demonstration that components produced by different manufacturers exhibit differing wear rates and have different molecular weights.
In the compression moulding of powders of high molecular weight such as UHMWPE, once the powder particle surfaces are in intimate contact, homogenisation requires interdiffusion of the polymer chains across the interfaces. This occurs by the motion of entangled polymer chains along tortuous paths defined by neighbouring molecules: a process known as reptation. When molecules of given molecular weight have reptated to the extent that their new shapes have no correlation with their original shapes, they are said to have reptated. The time taken for polymer chains to reptate increases strongly with molecular weight and a typical sample of UHMWPE is normally believed to contain a wide spectrum of molecular weights. Thus, to maximise homogenisation, it is necessary to mould articles where as much as possible of the polymer has reptated, and hence the maximum reptated molecular weight is as high as possible.
The practical problem for UHMWPE and the moulding of prostheses from UHMWPE is that homogenisation, wetting and molecular diffusion take place exceptionally slowly in UHMWPE even in the molten state. Moreover, they are highly sensitive to temperature time history, and therefore in a typical moulding will occur to differing extents in different parts of a moulding. The problem is exacerbated by the need for supplying moulded components in different sizes, according to the needs of the patient.
The present invention provides a method by which the bonding of already compacted particles of a polymer throughout a moulding of arbitrary size and shape can be controlled. A finite-element model of the component is formed, the continuous solid being represented as a mesh of tetrahedral, brick-shape or other elements, joined together at their apices. The temperature at the surface of the model component is taken through a cycle of elevated temperature followed by fall in temperature. Conventional theory of heat conduction is used to calculate numerically the time/temperature history throughout the component. Standard finite-element packages can be used for these calculations.
The maximum reptated molecular weight may be calculated as a function of position and time within the moulding, according to the equations below. The controlled cycle of time/temperature at the surface and/or the component design is modified until the calculation of reptated molecular weight shows that a satisfactory maximum reptated molecular weight is achieved throughout the moulding.
According to the present invention there is provided a process of compression moulding an article from a polymer powder comprising:
a) applying a polymer powder to a mould;
b) compacting the polymer powder by the application of a pressure P;
c) raising the temperature of the surface of the mould to a value above the melting temperature of the polymer powder; and
d) maintaining a temperature TM at the surface of the mould for a period of time tM and then cooling to a temperature below the crystallisation temperature of the polymer;
where TM and tM are controlled so as to provide a moulded article comprising a polymer of predetermined maximum reptated molecular weight ({circumflex over (M)}).
Preferably, TM is controlled so as to produce a moulded article comprising a polymer where the final, maximum reptated molecular weight {circumflex over (M)}f(x) of the polymer exceeds a specified value at all locations in the article. Preferably, {circumflex over (M)}f(x) for any given position x within the moulded article, is determined according to the following formula:                     M        ^            f        ⁢          xe2x80x83        ⁢          (      x      )        =            (                                    D            *                    ⁢                      xe2x80x83                    ⁢                      M                          *              n                                ⁢                      xe2x80x83                    ⁢          ξ          ⁢                      xe2x80x83                    ⁢                      (                          x              ,                              t                f                                      )                          β            )              1              (                  1          +          n                )            
as discussed below where tf is the time at which the last point of the moulding is predicted to fall below a specified crystallisation temperature, D* is the self-diffusion coefficient for a reference polymer of molecular weight M*, xcex2 is a constant and n is defined by the relationship   n  =      -                            ∂                      xe2x80x83                    ⁢          ln                ⁢                  xe2x80x83                ⁢        D                              ∂                      xe2x80x83                    ⁢          ln                ⁢                  xe2x80x83                ⁢        M            
where D is the self-diffusion coefficient of a monodisperse polymer of molecular weight M, and "xgr" is an equivalent time at the reference temperature T* applying to D* calculated from the temperature-time history T(x,u)             ξ      ⁢              xe2x80x83            ⁢              (                  x          ,          t                )              =                  ∫        0        t            ⁢                        ⅆ          u                                      a            T                    ⁢                      xe2x80x83                    ⁢                      (                          x              ,              u                        )                                ⁢      xe2x80x83  
where aTxe2x88x921=0 until the calculated temperature rises above a specified melting temperature, and then       a    T          -      1        =      exp    ⁢          xe2x80x83        ⁢          (                        Q          R                ⁡                  [                                    1                              T                *                                      -                          1              T                                ]                    )      
until the calculated temperature falls below a specified crystallisation temperature; thereafter aTxe2x88x921=0.
{circumflex over (M)} (x,tM) is thus the maximum restated molecular weight calculated as a function of the position of and the time of the polymer in the mould.
As specified above, n is related to the self-diffusion coefficient D of a monodisperse polymer of molecular weight M by the equation:   n  =      -                            ∂                      xe2x80x83                    ⁢          ln                ⁢                  xe2x80x83                ⁢        D                              ∂                      xe2x80x83                    ⁢          ln                ⁢                  xe2x80x83                ⁢        M            
and n is to be determined from the appropriate experimental data. The best current determinations for the value of n is that it is in the range of from 2 to 2.5, in particular about 2.4.
Preferably, the polymer is polyethylene such as UHMWPE, and TM and tM are chosen to give {circumflex over (M)}f (x) in the range of from 3xc3x97106 g/mol to 6xc3x97106 g/mol. Other polymers may also be compression moulded according to the invention, for example ultrahigh molecular weight polypropylene or polytetrafluoroethylene.
Preferably, the pressure P is in the range 9 MPa to about 30 MPa.
Preferably, the process includes a cooling step, wherein the mould is cooled. The rate may be in the range of from 5-10 degrees Kelvin/min.
Also according to the invention there is provided a moulded article formed from a process of compression moulding a polymer powder comprising:
a) applying a polymer powder to a mould;
b) compacting the polymer powder by the application of a pressure P;
c) raising the temperature of the surface of the mould to a value above the melting temperature of the polymer powder; and
d) maintaining a temperature TM at the surface of the mould for a period of time tM and then cooling to a temperature below the crystallisation temperature of the polymer;
where TM and tM are controlled so as to provide a moulded article comprising a polymer of predetermined molecular weight ({circumflex over (M)}f(x)) greater than a specified value.
According to a further aspect of the invention, the moulded polymer article is an orthopaedic prosthesis. The article may be a component of a joint replacement for the treatment of arthritis and other degenerative diseases of the joints, particularly of the knee, hip, shoulder, elbow, wrist, ankle, finger and toe joints.
Preferably, TM is controlled to give a moulded article comprising the polymer where the final, maximum reptated molecular weight {circumflex over (M)} (x,tM) exceeds a specified value at all locations in the article. Preferably, {circumflex over (M)}f(x) is determined according to the following formula:                     M        ^            f        ⁢          xe2x80x83        ⁢          (      x      )        =            (                                    D            *                    ⁢                      xe2x80x83                    ⁢                      M                          *              n                                ⁢                      xe2x80x83                    ⁢          ξ          ⁢                      xe2x80x83                    ⁢                      (                          x              ,                              t                f                                      )                          β            )              1              (                  1          +          n                )            
as discussed below.
The temperature TM and the time tM for which this temperature is maintained at the mould surface, may be controlled to give a moulded article of a predetermined maximum reptated molecular weight.
Calculations are carried out to determine the specific time/temperature surface history required for every separate shape and size of component; these calculations result in a process that provides a moulded article of a polymer of maximum reptated molecular weight at least equal to a specified minimum value throughout the component.
As an example, calculations indicate that when the mould surface temperature is raised to 165xc2x0 C. for 10 minutes, it can take uc to 45 minutes for the centre of a simple cylindrical moulding of UHMWPE, thickness 15 mm, to reach its maximum reptated molecular weight of 4.25xc3x97106 g/Mol, whereas the surface of the moulding does so in 30 minutes.
The present invention provides moulded articles of high strength and wear resistance. As examples, the articles may be UHMWPE articles for use as orthopaedic prostheses. High strength and wear resistance are particularly important in this application, as failure of the UHMWPE element of a prosthesis through cracking, or excess cold flow, can lead to an immediate need for a revision operation. Steady wearing away of the UHMWPE element is more insidious, but the small wear particles released can be consumed by the bone cells, leading to a hostile reaction, the formation of osteoclasts, the development of osteolysis and the loosening of the prosthesis.
The bulk of the orthopaedic components in current use are manufactured by machining from segments cut from compression moulded slabs or ram-extruded rods. Individually compression moulded components are increasingly being used. In either event, assurance of sufficient reptation time and a satisfactory maximum reptated molecular weight is needed to avoid the consequences of inadequate fusion of the polymer powder particles into a solid in which memory of the inter-particle interfaces is retained as sites of potential crack formation.